Chemical Ionization Spectra Research Articles

Mass-spectra of new heterocycles: XXIV. Electron impact and chemical ionization study of <i>N</i>-[3-alkoxyand 3-(1<i>H</i>-pyrrol-1-yl)-2-thienyl]imidothiocarbamates

The behavior of a wide range of previously unknown N -(2-thienyl)imidothiocarbamates synthesized in a single preparative step from heterosubstituted allenes (methoxy-, 1-ethoxyethoxy- and 1 H -pyrrol-1-ylallenes) and aliphatic, cycloaliphatic, and aromatic isothiocyanates was studied under conditions of electron (70 eV) and chemical (reagent gas - methane) ionization. Under the electron impact, the studied compounds form an unstable molecular ion, the main direction of fragmentation of which is associated with the breaking of the C-N bond in the imidoformamide fragment with charge localization at the imine nitrogen atom. A similar decay channel of a molecular ion is also observed in the chemical ionization mass spectra of the studied compounds. In this case, characteristic, but low-intensity ions in the electron ionization spectra of N -(3-methoxy-2-thienyl)imidothiocarbamates become dominant in the chemical ionization spectra. In the chemical ionization spectra of N -[3-(1 H -pyrrol-1-yl)-2-thienyl]imidothiocarbamates, the maximum intensity peak belongs to the [ M - SMe]+ ion. For N -[3-(1-ethoxyethoxy)-2-thienyl]imidothiocarbamate, the peak of the ion formed upon successive elimination of ethoxyethene and methanethiol molecules from the [ M + H]+ ion has the maximum intensity.

An Arabidopsis GCMS chemical ionization technique to quantify adaptive responses in central metabolism.

We present a methodology to survey central metabolism in 13CO2-labeled Arabidopsis (Arabidopsis thaliana) rosettes by ammonia positive chemical ionization-gas chromatography-mass spectrometry. This technique preserves the molecular ion cluster of methyloxime/trimethylsilyl-derivatized analytes up to 1 kDa, providing unambiguous nominal mass assignment of >200 central metabolites and 13C incorporation rates into a subset of 111 from the tricarboxylic acid (TCA) cycle, photorespiratory pathway, amino acid metabolism, shikimate pathway, and lipid and sugar metabolism. In short-term labeling assays, we observed plateau labeling of ∼35% for intermediates of the photorespiratory cycle except for glyoxylate, which reached only ∼4% labeling and was also present at molar concentrations several fold lower than other photorespiratory intermediates. This suggests photorespiratory flux may involve alternate intermediate pools besides the generally accepted route through glyoxylate. Untargeted scans showed that in illuminated leaves, noncyclic TCA cycle flux and citrate export to the cytosol revert to a cyclic flux mode following methyl jasmonate (MJ) treatment. MJ also caused a block in the photorespiratory transamination of glyoxylate to glycine. Salicylic acid treatment induced the opposite effects in both cases, indicating the antagonistic relationship of these defense signaling hormones is preserved at the metabolome level. We provide complete chemical ionization spectra for 203 Arabidopsis metabolites from central metabolism, which uniformly feature the unfragmented pseudomolecular ion as the base peak. This unbiased, soft ionization technique is a powerful screening tool to identify adaptive metabolic trends in photosynthetic tissue and represents an important advance in methodology to measure plant metabolic flux.

Mapping Glyceride Species in Biodiesel by High-Temperature Gas Chromatography Combined with Chemical Ionization Mass Spectrometry.

Accurate and comprehensive identification of residual glycerides in biodiesel is an important part of fuel characterization due to the impact of glycerides on the fuel physicochemical properties. However, analysis of bound glycerol in biodiesel samples faces challenges due to lack of readily available standards of structurally complex glyceride species in nontraditional biodiesel feedstocks and a risk of misannotation in the presence of impurities in gas chromatographic separations. Here, we evaluate methane and isobutane chemical ionization-single quadrupole mass spectrometry combined with high-temperature gas chromatography separations for mapping monoacylglycerols, diacylglycerols, and triacylglycerols in biodiesel. Unlike electron impact ionization, which produces mostly in-source fragments, isobutane chemical ionization spectra of tetramethylsilyl-derivatized monoacylglycerols and diacylglycerols are dominated by molecular ions and M-SiO(CH3)3+ ions, which provide important diagnostic information. We demonstrate the utility of isobutane chemical ionization in identifying structurally complex glycerolipid standards as well as species in biodiesel samples from different plant and animal feedstocks.

Characterization of aerosol nitroaromatic compounds: Validation of an experimental method.

The analytical capabilities associated with the use of silylation reactions have been extended to a new class of organic molecules, nitroaromatic compounds (NACs). These compounds are a possible contributor to urban particulate matter of secondary origin which would make them important analytes due to their (1) detrimental health effects, (2) potential to affect aerosol optical properties, and (3) and usefulness for identifying PM2.5 from biomass burning. The technique is based on derivatization of the parent NACs by using N,O-bis-(trimethylsilyl)-trifluoro acetamide, one of the most prevalent derivatization reagent for analyzing hydroxylated molecules, followed by gas chromatography-mass spectrometry using electron ionization (EI) and methane chemical ionization (CI). This method is evaluated for 32 NACs including nitrophenols, methyl-/methoxy-nitrophenols, nitrobenzoic acids, and nitrobenzyl alcohols. Electron ionization spectra were characterized by a high abundance of ions corresponding to [M+ ] or [M+ -15]. Chemical ionization spectra exhibited high abundance for [M+ +1], [M+ -15], and [M+ +29] ions. Both EI and CI spectra exhibit ions specific to nitro group(s) for [M+ -31], [M+ -45], and [M+ -60]. The strong abundance observed for [M+ ] (EI), [M+ -15] (EI/CI), or [M+ +1] (CI) ions is consistent with the high charge stabilizing ability associated with aromatic compounds. The combination of EI and CI ionization offers strong capabilities for detection and identification of NACs. Spectra associated with NACs, containing hydrogen, carbon, oxygen, and nitrogen atoms only, as silylated derivatives show fragment/adduct ions at either (a) odd or (b) even masses that indicate either (a) odd or (b) even number of nitro groups, respectively. Mass spectra associated with silylated NACs exhibited 3 distinct regions where characteristic fragmentation with a specific pattern associated with (1) ─OH and/or ─COOH groups, (2) ─NO2 group(s), and (3) benzene ring(s). These findings were confirmed with applications to chamber aerosol and ambient PM2.5 .

GC‐MS Characterization of Hydroxy Fatty Acids Generated From Lipid Oxidation in Vegetable Oils

Lipid oxidation has long been described as following a radical chain reaction mechanism, where hydrogen abstraction is considered the preferred pathway. Hydroxy compounds are, in theory, major products formed from hydrogen abstraction but their presence is rarely monitored. In this study, a GC‐MS technique to characterize hydroxy fatty acids (FA) formed during the oxidation of sunflower and canola oils is described. First, hydroxy FA in oxidized oils are methylated and isolated from non‐oxygenated structures using solid phase extraction (SPE). Then they are converted into their trimethylsilyl (TMS) derivatives using a N,O‐bis (trimethylsilyl) trifluoroacetamide (BSTFA)‐pyridine method and identified by their electron ionization (EI) and positive chemical ionization (PCI) spectra. Separation of most isomeric hydroxy FA with very similar structures is accomplished using a DB‐23 capillary column with (50%‐cyanopropyl)‐methylpolysiloxane phase. The fragmentation patterns of the TMS derivatives are discussed in detail and several easily applicable rules for spectral interpretations are presented. The major hydroxy FA arising from oxidation of sunflower and canola oils are allylic and conjugated structures, specifically 8‐, 9‐, 10‐, and 11‐hydroxyoctadecenoic acid (OH‐C18:1) and 9‐ and 13‐hydroxyoctadecadienoic acid (OH‐C18:2).Practical Applications: Unsaturated hydroxy FA are potential indicators of hydrogen abstraction reactions by FA alkoxyl radicals during lipid oxidation. A prerequisite for the quantification of these hydroxy FA is knowledge of their structures. In this work a GC‐MS method, coupled with SPE and TMS derivatization, to characterize the hydroxy FA derived from oxidation of vegetable oils, which will enable future quantifications of these compounds is employed. In addition, the use of EI and PCI spectra provided clear strategies to interpret mass spectra for both saturated and unsaturated hydroxy FA. The fragmentation patterns in EI and PCI presented in this paper will benefit future studies on identification or quantification of both saturated and unsaturated hydroxy FA in other lipid samples.Hydroxy fatty acids in oils are methylated and isolated using solid phase extraction. They are converted into their trimethylsilyl (TMS) derivatives, which enabled identification by the electron ionization and positive chemical ionization spectra. Here, the fragmentation patterns of the TMS derivatives are discussed in detail and several easily applicable rules for spectral interpretations are presented.

Differentiation of the six dimethoxypyrovalerone regioisomers: GC-MS, GC-MS/MS and GC-IR

Multiple and complementary analytical methods are often necessary for the identification of a specific compound from a series of closely related structural isomers. Gas chromatography-mass spectrometry (GC-MS), gas chromatography-product ion mass spectrometry (GC-MS/MS) and gas chromatography-infrared spectroscopy (GC-IR) were used to differentiate between the six dimethoxypyrrovalerone (DMPV) regioisomers. The six regioisomeric aminoketones were separated on a 50% phenyl stationary phase and the elution order is related to the positioning of substituents on the aromatic ring. These six DMPV regioisomers yield essentially identical mass spectral data in both chemical ionization (CI-MS) and electron ionization (EI-MS) spectra as well as identical product ion MS/MS spectra of the iminium cation base peak (m/z 126). These various mass spectral techniques provide data to identify all major structural features of these molecules except the dimethoxy substitution pattern of the aromatic ring. The region of the vapor phase infrared spectra between 1600cm−1 and 1000cm−1 provides a significant number of unique absorption bands characteristic of each individual DMPV regioisomer.

Determination of triterpenoids from birch bark by liquid chromatography-tandem mass spectrometry

Pentacyclic triterpenoids from birch bark are of great interest as biologically active substances and raw materials for producing a wide range of drugs. In this work, a method for the highly sensitive determination of betulin, betulinic acid, lupeol, and erythrodiol in plant extracts based on a combination of high-performance liquid chromatography with tandem mass spectrometric detection is proposed. Mass spectra under the conditions of atmospheric pressure chemical ionization and collision-induced dissociation spectra of analytes are obtained. The conditions for mass spectrometric detection in the multiple reaction monitoring mode are optimized. The calculated detection limits are in the range 0.7–1.8 μg/L.

Gas Chromatography/Electron Ionization Mass Spectrometric Analysis of Oligomeric Polyethylene Glycol Mono Alkyl Ethers

Polyethylene Glycol Monoalkyl Ethers, CxH2x+1 (OC2H4)n OH, (PEGMAE), are polar compounds like Polyethylene Glycols (PEG) and they undergo microbial degradation which produces toxic substances that are potentially dangerous to the environment. Therefore, the purpose of this study is to carry out proper identification and characterization of these compounds. The Electron Ionization (EI) and Chemical Ionization (CI) spectra of various PEGMAE were obtained by Gas Chromatography/Mass Spectrometry (GC/MS) and were used to identify and characterize these compounds. The characteristic cleavages in the EI and CI reactions of PEGMAEs were also studied. The results obtained showed that the methane CI mass spectra of the PEGMAE contain MH + ions and fragment ions similar to those found in their EI mass spectra. The relative abundances of the MH + ions are low, variable and increase with increasing sample size across the chromatographic peaks; but do not increase with increasing x or n. The base peaks of the low mass oligomers (x3) are PEG related (e.g., m/z 45, 59) but those of higher mass oligomers (x4) include the ion at m/z 63 (HOC2H4OH) H + or m/z 107 (HO (C2H4O)2 H) H + . There are no (MH-H2O) + ions or protonated dimer ions (n2, x2) in the spectra of PEGMAE. The Relative Molar Sensitivities (RMS) or the Relative Sensitivity per Gram (RSG) increases linearly with molecular polarizabilty or molecular weight with a non-zero intercept.

Inhibition of Prostate Cancer (LNCaP) Cell Proliferation by Volatile Components from Nagami Kumquats

Fresh Nagami kumquats (Fortunella margarita) were subjected to hydrodistillation using a Clevenger-type apparatus to obtain volatile oil. The chemical composition of the volatile oil was analyzed by GC-MS using Rtx-5 Sil MS and DB Wax columns. A total of 25 volatile compounds were identified by mass spectra, retention index, and comparison with known standards. The major identified compounds are d-limonene (41.64 %), β-myrecene (16.54 %), linalyl propionate (9.55 %), and germacrene-D (5.93 %) from the Rtx-5 Sil MS column; d-limonene and β-myrecene were also separated as major compounds on the DB wax column. The oil is rich in hydrocarbons (77.41 %) consisting of 60.05 % monoterpenes and 17.36 % sesquiterpenes. Interestingly, oxygenated hydrocarbons (17.6 %) were also found in kumquat volatile oil. Certain volatile compounds were also confirmed by positive chemical ionization and NMR spectra. Further, the volatile oil demonstrated good DPPH radical scavenging activity and antioxidant capacity. Kumquat volatile oil at 200 ppm concentration exhibited 55 %, 61 %, and 63.4 % inhibition of human prostate cancer (LNCaP) cell proliferation at 24, 48, and 72 h, respectively, by cell count assays. Significant increases in expression of bax/bcl2 and p53 proteins confirmed that volatile oil induces apoptosis. In addition, inhibition of inflammatory markers such as NF-κB and Cox-2 was observed. The cleavage of caspase-8 in the LNCaP cells treated with volatile oil demonstrated that apoptosis occurred through an extrinsic pathway. This is the first report of the identification and possible mechanisms of in vitro antiproliferative effects of kumquat volatile components on human prostate cancer (LNCaP) cells.

Detection, isolation and characterization of normorphine and norcodeine as morphine metabolites in man.

Abstract Normorphine and norcodeine were identified by two-dimensional thin-layer chromatography (t.l.c.) as human urinary morphine metabolites from 24 h urine samples of three patients who had received 190–210 mg morphine sulphate intravenously and from one patient who had chronically received 220 mg morphine sulphate orally per day. Both metabolites were isolated by preparative t.l.c. Comparison of the isolated norcodeine and normorphine with their respective authentic compounds, prepared by chemical synthesis, showed identical melting points, mass fractionation patterns, and chemical ionization spectra, confirming t.l.c. identification.

Microsynthesis and mass spectral study of Chemical Weapons Convention related 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides

AbstractChemical Weapons Convention (CWC)‐related compounds where the phosphorus atom is part of a ring have very limited representation in mass spectral libraries and the open literature. Here we report electron ionization (EI), chemical ionization (CI) and electrospray ionization tandem mass spectrometry (ESI‐MSn) spectra and retention indices for 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides (alkyl C1 to C3) which are new cyclic chemicals covered under the CWC. The EI mass spectra show a pattern of ion fragmentation that is similar to that of other cyclic phosphonates in that loss of the alkylphosphonic acid as a neutral loss is more important than the presence of the protonated alkylphosphonic acid. In contrast to other cyclic phosphonates, the 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides show almost no protonated alkylphosphonic acid and as a result the spectra do not carry the same distinctive signature of the phosphorus–carbon bond that is required for the chemical to be covered under the CWC. Copyright © 2008 John Wiley & Sons, Ltd.

The intriguing case of organic impurities contained in synthetic methanol: a mass spectrometry based investigation

The role of organic impurities in the methanol-to-olefin (MTO) industrial process catalyzed by zeolites is the subject of ongoing debate. We have found that methanol (HPLC and RPE grade) purchased from different chemical companies may contain organic impurities, whose ionization is the dominant process in the positive ion atmospheric pressure chemical ionization (APCI) spectrum of commercial CH(3)OH. Such impurities produce ions with elemental formulae C(n)H(2n+1)O(+) (n = 4, 5, 6); likewise, ionization of tetradeuterated methanol (CD(3)OD) leads to the corresponding fully deuterated series C(n)D(2n+1)O(+) (n = 4, 5, 6), an outcome which represents a clear evidence of their widespread diffusion. We suggest that their formation might be inherent to the chemical process whereby methanol is synthesized on an industrial scale. Mass spectrometry (MS) experiments, gas chromatography/mass spectrometry (GC/MS) analysis and nuclear magnetic resonance (NMR) measurements allowed us to establish that commercial methanol contains dimethyl acetals of simple alkyl ketones, such as propanone, butanone and pentanone. Ab initio calculations (DFT/B3LYP) proved useful to understanding the ionization mechanisms of such impurities.

Supercritical fluid extraction and quantification of aflatoxins in Zizyphi Fructus by liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry

An integrated method combining supercritical fluid extraction (SFE) with liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry (LC/APCI-MS/MS) was developed and successfully applied to quantify aflatoxins (AFs) in Zizyphi Fructus (fruits of Zizyphus jujube), a traditional Chinese medicine. To minimize the potential interferences caused by the complex matrix in Zizyphi Fructus, a SFE pretreatment was performed. In addition, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) spectra were also compared. The results showed that the calibration curves of AFB(1), AFB(2), AFG(1), and AFG(2) were all linear over the range of concentration from 1 to 50 ng/g, the squared correlation coefficients (r(2)) were over 0.995, and the detection limits of the method were between 0.17 and 0.32 ng/g. It showed high recovery and good precision in quantitating AFs in Zizyphi Fructus without further clean-up. Further, fragmentation pathways of protonated AFs in APCI-MS/MS were clearly proposed which could predict the existence of AFB or AFG series. To test the empirical validity of the proposed methodology in this paper, eight random samples of Zizyphi Fructus collected from supermarkets and traditional Chinese medicine stores in different geographical areas of Taiwan were analyzed. The results indicated that low levels of AFs were detected in only one of them.

Determination of steryl sulphates in invertebrate tissue by liquid chromatography–tandem mass spectrometry

A method for the identification and quantification of underivatised steryl sulphates in invertebrates by liquid chromatography (LC) coupled with tandem mass spectrometry (MS) involving a single cleanup step has been developed. Negative electrospray ionisation and positive and negative atmospheric-pressure chemical ionisation (APCI) spectra of steryl sulphate showed pseudomolecular ions ([M+H-H2SO4]+ or [M-H]-). Collision-induced dissociation (CID) was efficient only in positive APCI. LC-MS in negative APCI was least susceptible to interference and possible differences in response factors. The detection limits (signal-to-noise ratio of 3) based on cholest-5-enyl-3-sulphate in positive and negative APCI modes are 3.66 and 0.73 pmol microL(-1), respectively. Calibration plots and response factors for cholest-5-enyl-3-sulphate relative to the internal standard, cholecalciferyl-3-sulphate, in both positive and negative polarities, were linear in the concentration range from 1.22 to 16.4 pmol microL(-1) with good coefficients of determination (R2 > 0.98). It is suggested that the structure elucidation of steryl sulphates is best achieved in CID positive APCI mode, whereas their quantification should be carried out using negative APCI.

Analysis of flame retardant additives in polymer fractions of waste of electric and electronic equipment (WEEE) by means of HPLC–UV/MS and GPC–HPLC–UV

An HPLC-UV/MS method has been developed to identify and quantify flame retardants in post-consumer plastics from waste of electric and electronic equipment (WEEE). Atmospheric pressure chemical ionisation spectra of 15 brominated and phosphate-based flame retardants were recorded and interpreted. The method was applied to detect flame retardant additives in polymer extracts obtained from pressurised liquid extraction of solid polymers. In addition, a screening method was developed for soluble styrene polymers to isolate a flame retardant fraction through the application of gel permeation chromatography (GPC). This fraction was transferred to an online-coupled HPLC column and detected by UV spectroscopy, which allowed a reliable qualitative and quantitative analysis of brominated flame retardants in the polymer solutions.

Direct coupling of a nano‐high‐performance liquid chromatography column to an ion trap designed for a gas chromatography/mass spectrometry system

AbstractThe possibility of direct liquid injection inside an ion trap, designed for a commercial gas chromatography/mass spectrometry (GC/MS) system, has been tested by continuous solution flow and by loop injection. Instrumental parameters such as solution flows and sample concentration, and temperatures of transfer line, ion trap and ion trap housing, have been optimised. Different configurations of the interface capillary have been also considered, and the best results have been obtained by the injection of the solution coaxially with a helium flow. The experimental arrangement leads to the detection of good quality electron (EI) and chemical (CI) ionisation spectra of low to medium polarity compounds, at µM levels. The direct coupling of a nano‐HPLC column with the modified transfer line led on one hand to EI spectra which were successfully employed for library searches, and on the other to CI data directly comparable to results achieved by a commercial HPLC/MS system operating under ESI conditions. Copyright © 2001 John Wiley & Sons, Ltd.

Study of Ion—Molecule Reactions and Collisionally-Activated Dissociation of Dopamine and Adrenaline by an Ion Trap Mass Spectrometer with An External Ionization Source

Study of the reaction mechanisms for ion–molecule reactions and for collisionally-activated dissociations (CAD) of dopamine and adrenaline has been performed using an external chemical ionization source quadrupole ion trap mass spectrometer. This work demonstrates the possibility of applying an external source ion trap instrument to perform selective ion–molecule reactions in the gas phase, due to its high sensitivity and low detection limits in mass spectrometry/mass spectrometry (MS/MS) mode. CAD experiments on ions with relative intensity as low as 0–2%, formed as ion–molecule products of dopamine and adrenaline, have been successfully performed. Study of some fragment ions of M+• and [M + H]+, observed in the chemical ionization (CI) spectra, by CAD techniques, permits elucidation of a series of mechanisms for the sequential dissociations of the M+• and [M + H]+ ions. Thus, the structural information obtained from this method is similar to that which would have been obtained if MS n had been performed for M+• and [M + H]+ ions. From the proposed CAD reaction mechanisms and the semi-empirical calculations, the favored reactive sites for formation of the adduct ions could be determined. The reactive site for protonation of dopamine is on the amino group, but for adrenaline, it is on the benzylic hydroxyl group. As to the reactive site for the CH3O=C2H+ ion addition, dopamine is either on the amino group or on the phenyl ring. However, adrenaline is only on the benzylic hydroxyl group. Temperature effects on the formation of the ion–molecule products were also investigated. It was shown that the best source temperature for formation of [M + H]+ and [M + 13]+ ions of dopamine is 200°C. Information about use of dimethyl ether (DME) as the reagent gas in the external chemical ionization of an ion trap mass spectrometer is provided.

Quantitation of monosaccharide isotopic enrichment in physiologic fluids by electron ionization or negative chemical ionization GC/MS using di-O-isopropylidene derivatives.

The aldonitrile pentaacetate and other derivatives lack ions in the electron ionization (EI) spectra possessing an intact hexose structure and thus must be analyzed by chemical ionization GC/MS in order to study multiple isotopomers. We report methods for quantitation of hexose di-O-isopropylidene acetate (IPAc) or pentafluorobenzoyl (PFBz) esters. These were prepared in a two-step procedure using inexpensive reagents that do not adversely impact the isotopomer structure of the sugar. The acetate derivative possesses an abundant [M - CH3] ion in the EI spectrum which is suitable for quantitative analysis of isotopomers. The negative chemical ionization (NCI) spectrum of the corresponding pentafluorobenzoyl derivative has a dominant molecular anion. Moreover, the PFBz derivative is about 100-fold more sensitive than the acetate, which offers some advantages for analysis of minor hexoses found in plasma. Isotopic calibration curves of [U-13C]glucose are linear over the 0.1-60% tracer/tracee range tested. The useful range for isotopic tracer studies is 25-2500 pmol for EI analysis of the acetate derivative and 0.1-55 pmol for NCI analysis of PFBz derivative (sample amount injected). For most studies where sample size is not limited, EI-GC/MS analysis of the IPAc derivative is preferred. NCI-GC/MS analysis is reserved when sample size is limiting or when studies involve hexoses other than glucose that are normally present at low concentration.

Mass spectral study of substituted allyl aryl and allyl alkyl selenides and some analogous sulfides

The electron impact (EI) mass spectra of allyl aryl selenides showed abundant molecular ions and many fragment ions containing the selenium atom. alpha-Cleavage is the dominant process in the fragmentation of selenides, and cleavage product ions are characteristic of the substituents. In the case of 3-methyl allyl and related aryl selenides, characteristic delta-hydrogen migration to the selenium atom is observed. A McLafferty-type rearrangement is found in benzyl allyl selenides and substituted alkyl allyl selenides. The charge on the rearrangement products preferably remains on the fragments containing the phenyl group. The [M - SeH](+), [M - CH(3)](+) and [M - C(2)H(4)](+) ions are found only in the EI mass spectrum of allyl phenyl selenide, and are attributed to a Claisen rearrangement in the source of the mass spectrometer. All structurally informative fragmentation processes are supported by collision induced dissociation spectra of molecular ions. The fragmentation patterns found in methane chemical ionization (CI) spectra of the selenides were significantly different from those observed in EI. The EI and CI mass spectra of analogous sulfides showed similar behaviour to that observed in the corresponding selenides. Copyright 1999 John Wiley & Sons, Ltd.

Mass Spectra of Cyclic Amidines and Their Salts. Part II. 2-Aryl and L, 2-Diaryl-l, 4, 5, 6-Tetrahydropyrimidines and Their Corresponding Methiodides

Mass spectra of a series of 2-mono- and 1, 2-disubstituted- 1, 4, 5, 6-tetra hydropyrimidines 1 and their methiodides 2 under electron impact (EI) are analyzed. A typical fragmentation pattern was found for each series of compounds. One of the fragmentation routes proposed for compounds 1 was confirmed by high resolution techniques. A fast atom bombardment (FAB+) technique was employed in order to detect methiodides 2 molecular ion. The chemical ionization (CI) spectrum of compound 2d is also analyzed.